The specific aims of this project are: 1) to determine the rates of fluid- phase endocytosis (pinocytosis) in replicating myoblasts, perfusion nonreplicating myoblasts, myotubes, and several developmentally defective mutant cell lines, 2) to determine the pattern of developmental regulation of receptor-mediated endocytosis of several proteins and peptide hormones during myogenesis, using several monoclonal antibodies directed against antigens that are suspected of involvement in receptor-mediated endocytosis, 3) to investigate the characteristics of two cell lines, fu- 103 and fu-104, which are not as defective in their development as fu-15 cells. Reactivity with a bank of monoclonal antibodies, growth characteristics, and fusion capabilities will be determined. The long-term objective of this research plan is to test the hypothesis that fluid-phase endocytosis and receptor-mediated endocytosis of many ligands are drastically down-regulated shortly before the irreversible commitment to terminal differentiation in myogenesis. Since cells of myogenic and other lineages withdraw from the cell cycle as part of their development, it is expected that they will specifically down-regulate cell surface receptors for many growth factors, becoming refractory to their effects. They may also reduce endocytic levels in general. Several congenital defects, including some forms of muscular dystrophy, are related to improper development. This project addresses fundamental mechanisms involved in normal and abnormal development.